In an electronic circuit, in a case a plurality of signals at different frequencies are input, so-called intermodulation distortion arises a problem. For example, a case of an amplifier circuit provided at an input stage of a receiving circuit is typical. For example, in a case the difference of two carrier frequencies is present near the frequency of a desired wave, there is a problem of “intermodulation distortion” where an interfering component is also demodulated. Typically, when signals at a plurality of frequencies adjacent to the receiving band of a self station are received, if the linear performance of an amplifier circuit or a frequency mixer circuit is poor, third-order distortion is caused within the receiving band (normally, only a first-order component of a modulated signal has to be taken into account), and the reception quality is significantly reduced.
As a method of preventing the problem of “intermodulation distortion”, there is known a method of adding a wavelength selective bandpass filter to an input unit of a receiving circuit, for example. However, this method increases the cost regarding the bandpass filter, or increases the size of the substrate, for example. Also, since generally the bandpass filter acts only with respect to a fixed frequency, using the same while making the corresponding frequency variable is difficult, and the bandpass filter has to be prepared for each communication channel (in other words, a carrier frequency; hereinafter also referred to as a “band”).
As another method of preventing the problem of intermodulation distortion, there is also known a method of improving “non-linear operation of a circuit member” which is the cause of occurrence in the first place. This is a method where a circuit member is not added. For example, to improve the linear performance of a circuit, a measure of increasing the bias current or optimizing the DC bias point such that operation is in a linear region as much as possible is effective, but this increases the power supply voltage or increases the consumption power. Or, it is also conceivable to use an expensive circuit member with improved linearity, but even if an expensive circuit member is used, the non-linearity cannot, in principle, be made zero.
In contrast, for example, Japanese Patent Application Laid-Open No. 2000-244353 proposes a technology which assumes that occurrence of intermodulation distortion is inevitable, and which includes a gain control circuit (a variable gain amplifier circuit) at an input stage to restrict the gain at the time of large input such that the intermodulation distortion may be suppressed in a use state, to thereby suppress distortion at the gain control circuit or at a frequency conversion circuit (a mixer circuit, a mixer) at a later stage.
However, the technology proposed in Japanese Patent Application Laid-Open No. 2000-244353 includes two stages of gain control circuits in a system for input signals, and includes a level detection circuit (a wave detector) for each gain control circuit, and thus, the circuit is complicated.